Formic acid, H(CHO2), carbonic acid, HO(CHO2), and oxalic acid, (CHO2)2, are the simple organic or carboxylic acids, R(CHO2) corresponding with HH and HOH. Commencing with carbonic oxide, CO, the formation of carboxylic acids is clearly seen from the fact that CO is capable of combining with X2, that is of forming COX2. If, for instance, one X is an aqueous residue, OH (hydroxyl), and the other X is hydrogen, then the simplest organic acid—formic acid, H(COOH)—is obtained. As all hydrocarbons (Chapter VIII.) correspond with the simplest, CH4, so all organic acids may be considered to proceed from formic acid.
In a similar way it is easy to explain the relation to other compounds of carbon of those compounds which contain nitrogen. By way of an example, we will take one of the carboxyl acids, R(CO2H), where R is a hydrocarbon radicle (residue). Such an acid, like all others, will give by combination with NH3 an ammoniacal salt, R(CO2NH4). This salt contains the elements for the formation of two molecules of water, and under suitable conditions by the action of bodies capable of taking it up, water may in fact be separated from R(CO2NH4), forming by the loss of one molecule of water, amides, RCONH2, and by the loss of two molecules of water, nitriles, RCN, otherwise known as cyanogen compounds or cyanides.40 If all the carboxyl acids are united not only by many common reactions but also by a mutual conversion into each other (an instance of which we saw above in the conversion of oxalic acid into formic and carbonic acids) one would expect the same for all the cyanogen compounds also. The common character of their reactions, and the reciprocity of their transformation, were long ago observed by Gay-Lussac, who recognised a common group or radicle (residue) cyanogen, CN, in all of them. The simplest compounds are hydrocyanic or prussic acid, HCN, cyanic acid, OHCN, and free cyanogen, (CN)2, which correspond to the three simplest carboxyl acids: formic, HCO2H, carbonic, OHCO2H, and oxalic, (CO2H)2. Cyanogen, like carboxyl, is evidently a monatomic residue and acid, similar to chlorine. As regards the amides RCONH2, corresponding to the carboxyl acids, they contain the ammoniacal residue NH2, and form a numerous class of organic compounds met with in nature and obtained in many ways,41 but not distinguished by such characteristic peculiarities as the cyanogen compounds.
The reactions and properties of the amides and nitriles of the organic acids are described in detail in books on organic chemistry; we will here only touch upon the simplest of them, and to clearly explain the derivative compounds will first consider the ammoniacal salts and amides of carbonic acid.
As carbonic acid is bibasic, its ammonium salts ought to have the following composition: acid carbonate of ammonium , H(NH 4 )CO 3 , and normal carbonate , (NH 4 ) 2 CO 3 ; they represent compounds of one or two molecules of ammonia with carbonic acid. The acid salt appears in the form of a non-odoriferous and (when tested with litmus) neutral substance, soluble at the ordinary temperature in six parts of water, insoluble in alcohol, and obtainable in a crystalline form either without water of crystallisation or with various proportions of it. If an aqueous solution of ammonia be saturated with an excess of carbonic anhydride, and then evaporated over sulphuric acid in the bell jar of an air-pump, crystals of this salt are separated. Solutions of all other ammonium carbonates, when evaporated under the air-pump, yield crystals of this salt. A solution of this salt, even at the ordinary temperature, gives off carbonic anhydride, as do all the acid salts of